1. Field of the Invention
The present invention relates to a catalyst and to a method for producing a carboxylic acid and/or a carboxylic anhydride through vapor-phase partial oxidation of an organic compound in the presence of the catalyst by use of an oxygen-containing gas.
2. Background Art
There has already been known method for producing carboxylic acids through vapor-phase partial oxidation of an organic compound in the presence of a catalyst containing diamond and a Group 5 transition element oxide. Ind. Eng. Chem. Res., 32, 263-273 (1993) discloses a method for growing a porous diamond layer on a metal oxide surface through chemical vapor deposition. The non-patent document describes that the selectivity of the catalytic reaction to form phthalic anhydride through vapor-phase partial oxidation of o-xylene is estimated to be enhanced to a certain extent, based on the simulation results using a specific mathematical model and specific parameters, assuming that a diamond layer can be grown in a vanadium pentoxide (V2O5) surface without impairing its oxidation catalytic activity. The document also discloses that CVD growth of the diamond layer is performed under high-temperature/low-pressure conditions (400 to 500° C. and 25 Torr) in a methane-hydrogen mixture (reducing gas) atmosphere. Meanwhile, under such conditions, vanadium pentoxide is known to be rapidly reduced to low-valence vanadium oxides (see Kogyo Kagaku Zassi, 55, p. 68 (1952), and Nippon Kagaku Zassi 82, p. 276 (1961)), and is known to exhibit insufficient oxidation catalytic activity (see Catalyst, 8, p. 302 (1966)). In other words, in practice, CVD growth disclosed in the aforementioned non-patent document encounters difficulty in producing a vanadium pentoxide catalyst bearing a diamond layer and having a sufficient oxidation catalytic activity. In addition, CVD must be performed by means of a particular processing apparatus with low process efficiency, making CVD a catalyst production method of limited effectiveness. Ind. Eng. Chem. Res., 32, 263-273 (1993) does not address addition of a transition metal element oxide other than vanadium oxide, addition method of a diamond other than CVD, or the morphology of a diamond other than layered structure by CVD growth.
There have already known a large number of methods for producing carboxylic acids through vapor-phase partial oxidation of an organic compound by use of an oxygen-containing gas in the presence of a catalyst containing a Group 5 transition element oxide (particularly vanadium pentoxide), a Group 4 transition element oxide, and a Group 6 transition element oxide. Examples of vapor-phase partial oxidation of a lower hydrocarbon compound or a lower oxygen-containing organic compound include production of acetic acid from butene in the presence of a catalyst such as MoO3—WO3—V2O5 catalyst (see German Patent No. 2,040,455); production of maleic anhydride from a linear C4 compound such as n-butane in the presence of a catalyst such as V2O5—P2O5—TiO2 catalyst (see Japanese Patent 1976-95990A); and production of maleic anhydride from benzene in the presence of a catalyst such as V2O5—WO3—P2O5—TiO2 catalyst (see German Patent No. 1,141,343).
There has already been known vapor-phase partial oxidation of an aromatic compound having a substituent by use of an oxygen-containing gas. Specific examples include production of benzoic acid from toluene in the presence of a catalyst such as V2O5—TiO2—TeO2—Sb2O3 catalyst (see Japanese Patent 1993-255181A); production of phthalic anhydride from o-xylene in the presence of a catalyst such as V2O5—TiO2—Nb2O5—P2O5—K2O catalyst (see Japanese Patent 1974-41036B); production of phthalic anhydride from naphthalene in the presence of a catalyst such as V2O5—Nb2O5—TiO2—P2O5 catalyst (see Japanese Patent 1984-1378B); production of pyromellitic dianhydride from 1,2,4,5-tetraalkylbenzene including durene in the presence of a catalyst such as V2O5—TiO2—MoO3—P2O5 catalyst (see Japanese Patent 1970-15018B), V2O5—TiO2—Ag2O—MoO3—P2O5—CaO catalyst (see Japanese Patent 1995-171393A), or a layered catalyst of V2O5—MoO3—P2O5—Ag2O and V2O5—TiO2-rare earth metal oxide-P2O5—CeO2 (see Japanese Patent 2000-1484A) and production of pyromellitic dianhydride from 2,4,5-trialkylbenzaldehyde in the presence of a catalyst such as V2O5—TiO2—P2O5— (Sb2O5, Cs2O) catalyst (see Japanese Patent 1995-2864A) or V2O5—TiO2—Ag2O—MoO3—P2O5 catalyst (see Japanese Patent 2002-105078A and 2002-105079A).
As described above, the aforementioned patent documents disclose use of a catalyst containing a Group 5 transition element oxide (particularly vanadium pentoxide) with a Group 4 transition element oxide and/or a Group 6 transition element oxide in the production of carboxylic acids through vapor-phase partial oxidation of an organic compound by use of an oxygen-containing gas. However, these patent documents do not mention a similar catalyst containing diamond.
Those catalysts disclosed in the patent documents improve catalytic activity, reaction selectivity, and stability in performance to satisfactory technical levels. However, there is still demand for a more effective catalyst. In particular, in partial oxidation, high reaction selectivity is demanded. In order to improve the selectivity, investigations have generally been performed on additives to the catalyst. Thus, catalysts attaining high selectivity generally contain a large number of components. Such catalysts raise problems in that preparation thereof including blending a number of starting materials and preliminary treatments, is cumbersome, and that the ranges of catalyst composition and reaction conditions for attaining optimum reaction results are limited. In order to solve the problems, it would be advantageous to find catalyst additives which are readily usable and are adaptable to wide ranges of catalyst preparation conditions and reaction conditions.
Meanwhile, vapor-phase partial oxidation of an organic compound by use of an oxygen-containing gas is known to be a vigorously exothermic reaction involving complete combustion. Therefore, there have been widely employed an approach that a catalyst composition containing a Group 5 transition element oxide is caused to be supported on a carrier which is inert to the relevant reaction, to thereby disperse heat. In fact, Japanese Patent 1982-105241A and 1986-28456A disclose self-sintered shaped carriers made of high-purity silicon carbide serving as suitable carriers. However, since such carriers are produced in an inert (non-oxidizing) gas (e.g., nitrogen) atmosphere via a sintering step at very high temperature, the production cost problematically increases. Among silicon carbide carriers, those having a low-purity and containing silica are inexpensive and can be produced in a simple manner through calcinating in air at low temperature. However, such a silicon carbide carrier encounters difficulty in exhibiting good reaction results. Therefore, finding catalyst additives which enable use of inexpensive carriers would be of great industrial value.
Apart from the development of the aforementioned catalyst carriers, development of catalyst itself; i.e., an approach in which a substance which disperse heat is added to a catalyst composition containing a Group 5 transition element oxide has been studied. For example, Japanese Patent 1996-318160A, U.S. Pat. No. 6,660,681, and WO 2000/62926 (pamphlet) disclose such additives; granular silicon carbide; silicon nitride, boron nitride, and aluminum nitride; and granular β-silicon carbide, respectively. However, these patent documents do not disclose a catalyst composition or a catalyst containing diamond.